Homeostasis and Hormones
Homeostasis is the maintainance of a constant internal environment in response to a changing external environment. Hormones have an important role in this system. Hormones are made of proteins, they are released by glands, in to the blood stream, where they reach target cells. A specific hormone will fit a specific receptor protein, and this brings about a change in that cell.
Negative feedback is the mechanism by which the body maintains conditions within particular limits. The body will do this by opposing a change that deviates from the normal. The diagram below helps to explain this using the example of body temperature.
Note, that the opposite change takes the level too far below the normal, therefore a negative response back up will occur, and the process repeats itself, so that over time the temperature oscillates about the normal, within small limits.
It is important to maintain a constant temperature so that living organisms can maintain metabolism. There are two types of heat regulation: endothermic where the species controls their own temperature (mammals, birds), and ectothermic where temperature reflects the environmental temperature (lizards, fish).
Temperature in mammals is detected by thermoreceptors in the skin and hypothalymus which is in the brain. Changes in temperature bring about nerve impulses from the brain to the muscles and glands which will bring about changes depending on whether it is hot or cold.
The amount of glucose in your blood is carefully controlled. Again, this uses the hormonal system. The hormones responsible for regulating blood glucose are produced in the pancreas in particular areas called islets of Langerhans.
After you have eaten a meal, the blood glucose levels will begin to rise because because the carbohydrates in the food are digested and absorbed. This rise is detected by beta cells, which then will produce more insulin. This insulin then binds to receptor proteins in cell membranes (particularly in the liver). This causes more channel channels to open so that more glucose can enter the cell. As well as this, insulin encourages enzymes to convert glucose to glycogen (glycogenesis) for storage.
If however, you have been doing alot of exercise, and glucose is being used up, then alpha cells will produce glucagon, this causes the release of an enzyme that breaks glycogen to glucose (glycogenolysis [gli-ko-jen-oh-li-sis]).